1. Field of the Invention
The present invention relates generally to frequency modulated (FM) demodulators, and more particularly to a threshold extension device for improving the operating performance of such demodulators for input signals having relatively low signal-to-noise ratios.
2. Description of the Prior Art
FM communication receivers are sometimes employed to receive extremely weak incoming frequency modulated radio signals and to recover therefrom the intelligence or information originally transmitted from a distant FM transmitter. A good example of this occurs in the case of the down-link or space-to-earth portion of a spacecraft communication system where the spacecraft is at a lunar distance or greater with respect to a ground station receiver located on the earth and where space and power limitations aboard the spacecraft dictate that the transmitter be of relatively low power. Since the low power transmitter and the vast intervening distance separating the spacecraft and the ground station receiver combine to cause the radio signals received at the ground station to be extremely weak, the ground station receiver must be capable of processing the signal without superimposing thereon any appreciable amount of electrical circuit noise if the transmitted intelligence is to be recovered with a minimum of degradation.
One of the factors having a substantial effect on the operational capability of the communications receiver is the operating performance of the FM demodulator or demodulators used in the receiver. As the received signal becomes weaker, a threshold point is reached below which the demodulator usually begins to itself introduce electrical noise of noticeable and objectionable character. A large measure of such noise is due to click type noise impulses which are generated by the demodulation process itself, and which are a result of the interaction between randomly varying electronic noise and the signal input to the demodulator. More particularly, when operating below a certain threshold level, the signal amplitude will often be of the same order of magnitude as some of the random noise fluctuations within the demodulator and as a result, the noise components will combine with the signal components and produce a sudden 360.degree. phase excursion in the demodulator output and an undesirable click impulse will be impressed upon the output of the demodulator. Such click impulses are particularly undesirable in that they have substantial low frequency content which is not suppressed by the conventional demodulator output filters.
The prior art approach toward solving the problem of click impulses has been to provide means responsive to the demodulator output for detecting the presence of the click impulses and then removing the click impulses from the output signal. An example of one such technique is disclosed in U.S. Pat. No. 3,588,705, entitled "Frequency Modulation Demodulator Threshold Extension Device," wherein amplitude level detectors are used to detect the existence of clicks by examining the amplitude characteristics of the demodulated signal. Such an approach does not, however, completely solve the problem since any click impulses which do not reach the predetermined detection levels will not be detected and eliminated from the demodulated output signal.